Ion implanters are widely used in semiconductor manufacturing to selectively alter conductivity of materials. In a typical ion implanter, ions generated from an ion source are directed through a series of beam-line components which include one or more analyzing magnets and a plurality of electrodes. The analyzing magnets select desired ion species, filter out contaminant species and ions having incorrect energies, and adjust ion beam quality at a target wafer. Suitably shaped electrodes can be used to modify the energy and the shape of an ion beam.
In production, semiconductor wafers are typically scanned with an ion beam. As used hereinafter, “scanning” of an ion beam refers to the relative movement of an ion beam with respect to a wafer or substrate surface.
An ion beam is typically either a “spot beam” having an approximately circular or elliptical cross section or a “ribbon beam” having a rectangular cross section. For the purpose of the present disclosure, a “ribbon beam” may refer to either a static ribbon beam or a scanned ribbon beam. The latter type of ribbon beam may be created by scanning a spot beam back and forth at a high frequency.
In the case of a spot beam, scanning of a wafer may be achieved by sweeping the spot beam back and forth between two endpoints to form a beam path and by simultaneously moving the wafer across the beam path. Alternatively, the spot beam may be kept stationary, and the wafer may be moved in a two-dimensional (2-D) pattern with respect to the spot beam. In the case of a ribbon beam, scanning of a wafer may be achieved by keeping the ribbon beam stationary and by simultaneously moving the wafer across the ribbon beam. If the ribbon beam is wider than the wafer, a one-dimensional (1-D) movement of the wafer may cause the ribbon beam to cover the entire wafer surface. The much simpler 1-D scanning makes a ribbon beam a desired choice for single-wafer ion implantation production.
However, just like spot beams, ribbon beams can suffer from intrinsic non-uniformity problems. A ribbon beam typically consists of a plurality of beamlets, wherein each beamlet may be considered, conceptually, as one spot beam. Though beamlets within a ribbon beam travel in the same general direction, any two beamlets may not be pointing in exactly the same direction. In addition, each beamlet may have an intrinsic angle spread. As a result, during ion implantation with a ribbon beam, different locations on a target wafer may experience different ion incident angles. Furthermore, the beamlets may not be evenly spaced within the ribbon beam. One portion of the ribbon beam where beamlets are densely distributed may deliver a higher ion dose than another portion of the ribbon beam where beamlets are sparsely distributed. Therefore, a ribbon beam may lack angle uniformity and/or dose uniformity.
Although there have been attempts to improve either angle uniformity or dose uniformity of a ribbon beam, an efficient solution has not been made available for providing ribbon beams that meet both dose and angle uniformity requirements for ion implantation production. For example, it is typically required that a ribbon beam should produce, in a wafer plane, a dose uniformity with less than 1% variations together with an angle uniformity with less than 0.5° variations. Such stringent uniformity requirements are difficult to meet since both types of uniformity may be elusive.
These requirements tend to minimize the movement of electrons across a surface of a target wafer being implanted. Any such movement may lead to the generation of substantial local potential differences and implantation non-uniformities, which may in turn lead to electrical breakdowns between circuit elements. As a result, it has been found that when a magnetic field at a target wafer is substantially greater than that of the earth's magnetic field, non-uniform electron distributions may occur, resulting in increased breakdown and non-uniformity in dose distribution.
FIG. 1 depicts a common geometry 100 for implanting ions onto a target wafer. A ribbon beam 10, which typically exits from a mass selection slit (not shown), enters a magnetic deflector 101 at an entrance region. The magnetic deflector 101 deflects the incoming ribbon beam 10 to provide a mass-analyzed beam suitable for implantation of a target wafer 103 at an implantation station 102. In this specific geometry 100, a corrector-bar pair 104 may be introduced at the exit region of the magnetic deflector 101 to produce uniformity across the target wafer.
Referring to FIG. 2, the corrector-bar pair 104 includes a pair of horizontal magnetic core members, such as an upper steel bar 202 and a lower steel bar 204, that form a gap or space 206 to allow the ribbon beam 10 to pass therethrough. The corrector-bar pair 104 provides a magnetic supporting structure needed for producing desired deflection fields. A plurality of coils 208 may be wound along the upper steel bar 202 and the lower steal bar 204. Each coil 208 may be individually and/or independently excited with a current, so as to generate high-order multipole components without dedicated windings. Individual excitation of each coil 208, or each multipole, may deflect one or more beamlets within the ribbon beam 10. That is, local variations in ion density or shape of the ribbon beam 10 may be corrected by modifying the magnetic fields locally. These corrections may be made under computer control and on a time scale that is only limited by a decay rate of eddy currents in the horizontal magnetic core members 202, 204.
The distance 106 between the exit region of the magnetic deflector 101 and the target wafer 103, as depicted in FIG. 1, is typically quite short. As a result, the short distance 106 may cause a fringing field, which originates from the top of the magnetic deflector 101, to be undesirably intense.
One common solution for minimizing such fringing fields is achieved by integrating an additional component that provides magnetic field suppression into the geometry or design for ion implantation 100.
For example, FIG. 3 depicts a schematic of a magnetic clamp 300, which may be inserted immediately following the exit region of the magnetic deflector 101. The magnetic clamp 300 is essentially a rectangular box, including a first pair of horizontal steel plates 302 that are connected together by a second pair of vertical plates 304. The second pair of vertical plates 304 provides an effective technique for magnetically shorting the first pair of horizontal plates 302 together. The magnetic clamp 300 also includes an opening or channel 306 through which the ribbon beam 10 passes. Fringing magnetic flux B 308 originating from the pole of the magnetic deflector 101 enters the top horizontal steel plate 302 and exits through the bottom horizontal steel plate 302, causing fringing fields to be substantially attenuated. However, the use of a magnetic clamp in conjunction with a corrector-bar pair is inefficient and does not adequately solve the problems discussed above.
In view of the foregoing, it would be desirable to provide a technique for reducing magnetic fields at an implant location to overcome the above-described inadequacies and shortcomings.